1. Field of the Invention
The invention relates to microwave vacuum tube devices, in particular tubes modulated by a proximately positioned grid structure, i.e. gridded tubes.
2. Discussion of the Related Art
Microwave vacuum tube devices, such as power amplifiers, are essential components of many modern microwave systems including telecommunications, radar, electronic warfare and navigation systems. While semiconductor microwave amplifiers are available, they generally lack the power capabilities required by most microwave systems. Microwave vacuum tube amplifiers, in contrast, can provide higher microwave power by orders of magnitude. The higher power levels of vacuum tube devices are the result of the fact that electrons can travel at a much higher velocity in a vacuum with much less energy losses than in a solid semiconductor material. The higher speed of electrons permits a use of the larger structure with the same transit time. A larger structure, in turn, permits a greater power output, often required for efficient operations.
Microwave tube devices typically operate by introducing a beam of electrons into a region where the beam interacts with an input signal, and deriving an output signal from the thus-modulated beam. See, e.g., A. S. Gilmour, Jr., Microwave Tubes, Artech House, 1986, 191-313. Microwave tube devices include gridded tubes (e.g., triodes, tetrodes, and klystrodes), klystrons, traveling wave tubes, crossed-field amplifiers and gyrotrons. All require a source of emitted electrons. For example, a conventional klystrode 10 is shown in FIG. 1. The klystrode contains 5 main elements--a cathode 12, a grid 14, an anode 16, a tail pipe 18, and a collector 20. The whole tube is optionally placed in a uniform magnetic field for beam control. In operation, a RF voltage is applied between the cathode 12 and grid 14 by one of several possible circuit arrangements. For example, it is possible for the cathode to be capacitively coupled to the grid or inductively coupled with a coupling loop into an RF cavity containing the grid structure. The grid 14 regulates the potential profile in the region adjacent the cathode, and is thereby able to control the emission from the cathode.
The resulting density-modulated (bunched) electron beam 22 is accelerated toward the apertured anode 16 at a high potential. The beam 22 passes by a gap 19, called the output gap, in the resonant RF cavity and induces an oscillating voltage and current in the cavity. RF power is coupled from the cavity by an appropriate technique, such as inserting a coupling loop into the RF field within the cavity. Finally, most of the beam passes through the tail pipe 18 into the collector 20. By depressing the potential of the collector 20, some of the dc beam power can be recovered to enhance the efficiency of the device. Demonstrated efficiency of such devices is relatively high, e.g., reaching 50% at 1 GHz, and the typical gain is about 25 dB at 1 GHz.
The usual source of electrons for such microwave tube devices is a thermionic emission cathode, which is typically formed from tungsten that is either coated with barium or barium oxide, or mixed with thorium oxide. Thermionic emission cathodes must be heated to temperatures around 1000.degree. C. to produce sufficient thermionic electron emission current, e.g., on the order of amperes per square centimeter. (As used herein, thermionic cathode indicates a cathode that must be heated to at least 800.degree. C. to provide measurable emission.) The necessity of heating thermionic cathodes to such high temperatures creates several problems. For example, the heating limits the lifetime of the cathodes, introduces warm-up delays, requires bulky auxiliary equipment for cooling, and tends to interfere with modulation of emission in gridded tubes. The limited lifetime is due to the fact that the high operating temperatures cause constituents of the cathode, such as the barium or barium oxide, to evaporate from the hot surface. It is possible for the evaporated barium or barium oxide to then deposit onto the grid, which causes undesirable grid emission that essentially renders the device ineffective. Moreover, once the barium is depleted from the cathode, the cathode (and hence the tube) no longer functions. Many thermionic vacuum tubes therefore have operating lives of less than a year. The delay in emission is due to the time required for temperature ramp-up, and delays as long as four minutes are not uncommon. Such delays are unacceptable for many applications.
For gridded tubes, such as the klystrode 10 of FIG. 1, the high temperature environment near the grid electrode tends to introduce thermally induced geometrical and/or dimensional instability that changes the cathode-grid spacing, e.g., due to thermal expansion mismatch or structural sagging. These changes to the spacing tend to significantly interfere with the ability of the grid to modulate the cathode emission, and thus interfere with the overall operation of the gridded tube. Moreover, there is a certain minimum cathode-grid spacing that must be maintained, to ensure that such dimensional changes do not result in contact between the cathode and grid. Because of this minimum spacing requirement, it is not possible to move the cathode and grid closer together in order to decrease the cathode-grid transit time, which would in turn increase the maximum operating frequency of the device. For this reason, the frequency of gridded tubes with thermionic cathodes is limited.
Thus, there is a need for an improved electron source for microwave tube devices, particularly gridded tubes, which avoids problems of conventional devices and is able to reduce transit times.